Erythrocyte Glut1 Triggers Dehydroascorbic Acid Uptake in Mammals Unable to Synthesize Vitamin C

Functional characterization and modified rescue of novel AE1 mutation R730C associated with overhydrated cation leak stomatocytosis

We report the novel, heterozygous AE1 mutation R730C associated with dominant, overhydrated, cation leak stomatocytosis and well-compensated anemia. Parallel elevations of red blood cell cation leak and ouabain-sensitive Na(+) efflux (pump activity) were apparently unaccompanied by increased erythroid cation channel-like activity, and defined ouabain-insensitive Na(+) efflux pathways of nystatin-treated cells were reduced. Epitope-tagged AE1 R730C at the Xenopus laevis oocyte surface exhibited severely reduced Cl(-) transport insensitive to rescue by glycophorin A (GPA) coexpression or by methanethiosulfonate (MTS) treatment. AE1 mutant R730K preserved Cl(-) transport activity, but R730 substitution with I, E, or H inactivated Cl(-) transport. AE1 R730C expression substantially increased endogenous oocyte Na(+)-K(+)-ATPase-mediated (86)Rb(+) influx, but ouabain-insensitive flux was minimally increased and GPA-insensitive. The reduced AE1 R730C-mediated sulfate influx did not exhibit the wild-type pattern of stimulation by acidic extracellular pH (pH(o)) and, unexpectedly, was partially rescued by exposure to sodium 2-sulfonatoethyl methanethiosulfonate (MTSES) but not to 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) or 2-(trimethylammonium)ethyl methanethiosulfonate bromide (MTSET). AE1 R730E correspondingly exhibited acid pH(o)-stimulated sulfate uptake at rates exceeding those of wild-type AE1 and AE1 R730K, whereas mutants R730I and R730H were inactive and pH(o) insensitive. MTSES-treated oocytes expressing AE1 R730C and untreated oocytes expressing AE1 R730E also exhibited unprecedented stimulation of Cl(-) influx by acid pH(o). Thus recombinant cation-leak stomatocytosis mutant AE1 R730C exhibits severely reduced anion transport unaccompanied by increased Rb(+) and Li(+) influxes. Selective rescue of acid pH(o)-stimulated sulfate uptake and conferral of acid pH(o)-stimulated Cl(-) influx, by AE1 R730E and MTSES-treated R730C, define residue R730 as critical to selectivity and regulation of anion transport by AE1.

Progressive pseudogenization: vitamin C synthesis and its loss in bats

For the past 50 years, it was believed that all bats, like humans and guinea pigs, did not synthesize vitamin C (Vc) because they lacked activity of L-gulonolactone oxidase (GULO) in their livers. Humans and guinea pigs lack the activity due to pseudogenization of GULO in their genomes, but there is no genetic evidence to show whether such loss in bats is caused by pseudogenization. Unexpectedly, our successful molecular cloning in one frugivorous bat (Rousettus leschenaultii) and one insectivorous bat (Hipposideros armiger) ascertains that no pseudogenization occurs in these species. Furthermore, we find normal GULO protein expression using bat-specific anti-GULO polyclonal antibodies in bats, evaluated by Western blotting. Most surprisingly, GULO activity assays reveal that these two bat species have retained the ability to synthesize Vc, but at low levels compared with the mouse. It is known that bats in the genus Pteropus have lost GULO activity. We then found that functional constraints acting on the GULO of Pteropus vampyrus (which lost its function) are relaxed. These results imply that the ability to synthesize Vc in bats has not been lost completely in species as previously thought. We also suggest that the evolution of bat GULO genes can be a good model to study genetic processes associated with loss-of-function.

Variable 18F-fluorodeoxyglucose uptake in gastric cancer is associated with different levels of GLUT-1 expression

OBJECTIVES

Detection rates of gastric cancer in F-fluorodeoxyglucose (FDG)-PET depend on the histopathological characteristics of the primary tumor. To clarify this observation, FDG uptake in gastric carcinoma was analyzed by focusing on histopathology and on the expression of the glucose transporter (GLUT-1) in the primary tumor.

METHODS

Thirty-five patients with the diagnosis of gastric cancer underwent FDG-PET with visual image analysis and measurement of maximum standardized uptake value (SUV(max)) before surgical treatment. Resected tumor samples were categorized according to Union internationale contre le cancer, WHO, and Laurén classification and tumor differentiation. GLUT-1 expression was graded semiquantitatively by immunohistochemistry. Statistical analysis was done for the correlation of histology, different classifications, and tumor grading with SUV(max) and GLUT-1 expression.

RESULTS

SUV(max) significantly correlated with histopathological classifications according to the WHO (P=0.009) and Laurén classification (P=0.034). Signet-ring cell carcinoma had a median SUV(max) of only 3.0 (range, 1.0-11.5). Median SUV(max) for papillary and tubular carcinoma was 7.8 (range, 1.8-14.4). In 21 (60%) cases, GLUT-1 expression in the primary tumor was positive. GLUT-1 expression correlated significantly with tumor differentiation (P=0.018) and the classification according to Laurén (P=0.023) and WHO (P<0.001). Thirteen (76%) of 17 signet-ring cell carcinoma cases did not show any GLUT-1 expression. SUV(max) in relation to GLUT-1 expression showed a significant correlation (P=0.002). For cases with detectable GLUT-1 expression the median SUV(max) was 6.9 (range, 2.3-14.1) versus a median of 3.1 (range, 1-8.8) for cases without GLUT-1 expression.

CONCLUSION

FDG uptake in gastric cancer depends on GLUT-1 expression. One major reason for low FDG uptake in signet-ring cell carcinoma is the low GLUT-1 expression in this histological subtype of gastric cancer.

Mitochondrial GLUT10 facilitates dehydroascorbic acid import and protects cells against oxidative stress: mechanistic insight into arterial tortuosity syndrome

Mutations in glucose transporter 10 (GLUT10) alter angiogenesis and cause arterial tortuosity syndrome (ATS); however, the mechanisms by which these mutations cause disease remain unclear. It has been reported that in most cells, mitochondria are the major source of reactive oxygen species (ROS). Moreover, mitochondria are known to incorporate as well as recycle vitamin C, which plays a critical role in redox homeostasis, although the molecular mechanism(s) underlying mitochondrial vitamin C uptake are poorly understood. We report here that GLUT10 localizes predominantly to the mitochondria of smooth muscle cells and insulin-stimulated adipocytes, where GLUT10 is highly expressed. We further demonstrate that GLUT10 facilitates transport of l-dehydroascorbic acid (DHA), the oxidized form of vitamin C, into mitochondria, and also increases cellular uptake of DHA, which in turn protects cells against oxidative stress. This protection is compromised when GLUT10 expression in mitochondria is inhibited. In addition, we found that aortic smooth muscle cells from GLUT10-mutant mice have higher ROS levels than those from wild-type mice. Our results identify the physiological role of GLUT10 as the mitochondrial DHA transporter, and demonstrate that GLUT10 protects cells from oxidative injury. Furthermore, our findings provide a mechanism to explain the ascorbate in mitochondria and show how loss-of-function GLUT10 mutations may lead to arterial abnormalities in ATS. These results also reinforce the importance of vitamin C and ROS in degenerative diseases.

Cellular pathways for transport and efflux of ascorbate and dehydroascorbate

The mechanisms allowing the cellular transport of ascorbic acid represent a primary aspect for the understanding of the roles played by this vitamin in pathophysiology. Considerable research effort has been spent in the field, on several animal models and different cell types. Several mechanisms have been described to date, mediating the movements of different redox forms of ascorbic acid across cell membranes. Vitamin C can enter cells both in its reduced and oxidized form, ascorbic acid (AA) and dehydroascorbate (DHA), utilizing respectively sodium-dependent transporters (SVCT) or glucose transporters (GLUT). Modulation of SVCT expression and function has been described by cytokines, steroids and post-translational protein modification. Cellular uptake of DHA is followed by its intracellular reduction to AA by several enzymatic and non-enzymatic systems. Efflux of vitamin C has been also described in a number of cell types and different pathophysiological functions were proposed for this phenomenon, in dependence of the cell model studied. Cellular efflux of AA is mediated through volume-sensitive (VSOAC) and Ca(2+)-dependent anion channels, gap-junction hemichannels, exocytosis of secretory vesicles and possibly through homo- and hetero-exchange systems at the plasma membrane level. Altogether, available data suggest that cellular efflux of ascorbic acid - besides its uptake - should be taken into account when evaluating the cellular homeostasis and functions of this important vitamin.

A role for Na+/H+ exchangers and intracellular pH in regulating vitamin C-driven electron transport across the plasma membrane

Ascorbate (vitamin C) is the major electron donor to a tPMET (transplasma membrane electron transport) system that was originally identified in human erythrocytes. This plasma membrane redox system appears to transfer electrons from intracellular ascorbate to extracellular oxidants (e.g. non-transferrin-bound iron). Although this phenomenon has been observed in nucleated cells, its mechanism and regulation are not well understood. In the present study we have examined both facets of this phenomenon in K562 cells and primary astrocyte cultures. Using ferricyanide as the analytical oxidant we demonstrate that tPMET is enhanced by dehydroascorbate uptake via facilitative glucose transporters, and subsequent accumulation of intracellular ascorbate. Additionally, we demonstrate that this stimulation is not due to ascorbate that is released from the cells, but is dependent only on a restricted intracellular pool of the vitamin. Substrate-saturation kinetics suggest an enzyme-catalysed reaction across the plasma membrane by an as-yet-unidentified reductase that relies on extensive recycling of intracellular ascorbate. Inhibition of ascorbate-stimulated tPMET by the NHE (Na+/H+-exchanger) inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride, which is diminished by bicarbonate, suggests that tPMET activity may be regulated by intracellular pH. In support of this hypothesis, tPMET in astrocytes was significantly inhibited by ammonium chloride-pulse-induced intracellular acidification, whereas it was significantly stimulated by bicarbonate-induced intracellular alkalinization. These results suggest that ascorbate-dependent tPMET is enzyme-catalysed and is modulated by NHE activity and intracellular pH.

Relaxation of selection, niche construction, and the Baldwin effect in language evolution

Deacon has suggested that one of the key factors of language evolution is not characterized by an increase in genetic contribution, often known as the Baldwin effect, but rather by a decrease. This process effectively increases linguistic learning capability by organizing a novel synergy of multiple lower-order functions previously irrelevant to the process of language acquisition. Deacon posits that this transition is not caused by natural selection. Rather, it is due to the relaxation of natural selection. While there are some cases in which relaxation caused by some external factors indeed induces the transition, we do not know what kind of relaxation has worked in language evolution. In this article, a genetic-algorithm-based computer simulation is used to investigate how the niche-constructing aspect of linguistic behavior may trigger the degradation of genetic predisposition related to language learning. The results show that agents initially increase their genetic predisposition for language learning—the Baldwin effect. They create a highly uniform sociolinguistic environment—a linguistic niche construction. This means that later generations constantly receive very similar inputs from adult agents, and subsequently the selective pressure to retain the genetic predisposition is relaxed.

Unusual thermal disassembly of the SPFH domain oligomer from Pyrococcus horikoshii

Stomatin, prohibitin, flotillin, and HflK/C (SPFH) domain proteins are membrane proteins that are widely conserved from bacteria to mammals. The molecular functions of these proteins have not been established. In mammals, the domain is often found in raft-associated proteins such as flotillin and podocin. We determined the structure of the SPFH domain of PH0470 derived from Pyrococcus horikoshii using NMR. The structure closely resembles that of the SPFH domain of the paralog PH1511, except for two C-terminal helices. The results show that the SPFH domain forms stable dimers, trimers, tetramers, and multimers, although it lacks the coiled-coil region for oligomerization, which is a highly conserved region in this protein family. The oligomers exhibited unusual thermodynamic behavior, as determined by circular dichroism, NMR, gel filtration, chemical cross-linking, and analytical ultracentrifugation. The oligomers were converted into monomers when they were heated once and then cooled. This transition was one-way and irreversible. We propose a mechanism of domain swapping for forming dimers as well as successive oligomers. The results of this study provide what to our knowledge are new insights into the common molecular function of the SPFH domain, which may act as a membrane skeleton through oligomerization by domain swapping.

Stomatin-like protein-1 interacts with stomatin and is targeted to late endosomes

The human stomatin-like protein-1 (SLP-1) is a membrane protein with a characteristic bipartite structure containing a stomatin domain and a sterol carrier protein-2 (SCP-2) domain. This structure suggests a role for SLP-1 in sterol/lipid transfer and transport. Because SLP-1 has not been investigated, we first studied the molecular and cell biological characteristics of the expressed protein. We show here that SLP-1 localizes to the late endosomal compartment, like stomatin. Unlike stomatin, SLP-1 does not localize to the plasma membrane. Overexpression of SLP-1 leads to the redistribution of stomatin from the plasma membrane to late endosomes suggesting a complex formation between these proteins. We found that the targeting of SLP-1 to late endosomes is caused by a GYXXPhi (Phi being a bulky, hydrophobic amino acid) sorting signal at the N terminus. Mutation of this signal results in plasma membrane localization. SLP-1 and stomatin co-localize in the late endosomal compartment, they co-immunoprecipitate, thus showing a direct interaction, and they associate with detergent-resistant membranes. In accordance with the proposed lipid transfer function, we show that, under conditions of blocked cholesterol efflux from late endosomes, SLP-1 induces the formation of enlarged, cholesterol-filled, weakly LAMP-2-positive, acidic vesicles in the perinuclear region. This massive cholesterol accumulation clearly depends on the SCP-2 domain of SLP-1, suggesting a role for this domain in cholesterol transfer to late endosomes.

Model of the exofacial substrate-binding site and helical folding of the human Glut1 glucose transporter based on scanning mutagenesis

Transmembrane helix 9 of the Glut1 glucose transporter was analyzed by cysteine-scanning mutagenesis and the substituted cysteine accessibility method (SCAM). A cysteine-less (C-less) template transporter containing amino acid substitutions for the six native cysteine residues present in human Glut1 was used to generate a series of 21 mutant transporters by substituting each successive residue in predicted transmembrane segment 9 with a cysteine residue. The mutant proteins were expressed in Xenopus oocytes, and their specific transport activities were directly compared to that of the parental C-less molecule whose function has been shown to be indistinguishable from that of native Glut1. Only a single mutant (G340C) had activity that was reduced (by 75%) relative to that of the C-less parent. These data suggest that none of the amino acid side chains in helix 9 is absolutely required for transport function and that this helix is not likely to be directly involved in substrate binding or translocation. Transport activity of the cysteine mutants was also tested after incubation of oocytes in the presence of the impermeant sulfhydryl-specific reagent, p-chloromercuribenzene sulfonate (pCMBS). Only a single mutant (T352C) exhibited transport inhibition in the presence of pCMBS, and the extent of inhibition was minimal (11%), indicating that only a very small portion of helix 9 is accessible to the external solvent. These results are consistent with the conclusion that helix 9 plays an outer stabilizing role for the inner helical bundle predicted to form the exofacial substrate-binding site. All 12 of the predicted transmembrane segments of Glut1 encompassing 252 amino acid residues and more than 50% of the complete polypeptide sequence have now been analyzed by scanning mutagenesis and SCAM. An updated model is presented for the outward-facing substrate-binding site and relative orientation of the 12 transmembrane helices of Glut1.

Dynamic aspects of ascorbic acid metabolism in the circulation: analysis by ascorbate oxidase with a prolonged in vivo half-life

Because AA (L-ascorbic acid) scavenges various types of free radicals to form MDAA (monodehydroascorbic acid) and DAA (dehydroascorbic acid), its regeneration from the oxidized metabolites is critically important for humans and other animals that lack the ability to synthesize this antioxidant. To study the dynamic aspects of AA metabolism in the circulation, a long acting AOase (ascorbate oxidase) derivative was synthesized by covalently linking PEG [poly(ethylene glycol)] to the enzyme. Fairly low concentrations of the modified enzyme (PEG-AOase) rapidly decreased AA levels in isolated fresh plasma and blood samples with a concomitant increase in their levels of MDAA and DAA. In contrast, relatively high doses of PEG-AOase were required to decrease the circulating plasma AA levels of both normal rats and ODS (osteogenic disorder Shionogi) rats that lack the ability to synthesize AA. Administration of 50 units of PEG-AOase/kg of body weight rapidly decreased AA levels in plasma and the kidney without affecting the levels in other tissues, such as the liver, brain, lung, adrenal grand and skeletal muscles. PEG-AOase slightly, but significantly, decreased glutathione (GSH) levels in the liver without affecting those in other tissues. Suppression of hepatic synthesis of GSH by administration of BSO [L-buthionin-(S,R)-sulfoximine] enhanced the PEG-AOase-induced decrease in plasma AA levels. These and other results suggest that the circulating AA is reductively regenerated from MDAA extremely rapidly and that hepatic GSH plays important roles in the regeneration of this antioxidant.

Vitamin C: update on physiology and pharmacology

Although ascorbic acid is an important water-soluble antioxidant and enzyme cofactor in plants and animals, humans and some other species do not synthesize ascorbate due to the lack of the enzyme catalyzing the final step of the biosynthetic pathway, and for them it has become a vitamin. This review focuses on the role of ascorbate in various hydroxylation reactions and in the redox homeostasis of subcellular compartments including mitochondria and endoplasmic reticulum. Recently discovered functions of ascorbate in nucleic acid and histone dealkylation and proteoglycan deglycanation are also summarized. These new findings might delineate a role for ascorbate in the modulation of both pro- and anti-carcinogenic mechanisms. Recent advances and perspectives in therapeutic applications are also reviewed. On the basis of new and earlier observations, the advantages of the lost ability to synthesize ascorbate are pondered. The increasing knowledge of the functions of ascorbate and of its molecular sites of action can mechanistically substantiate a place for ascorbate in the treatment of various diseases.

A single conserved proline residue determines the membrane topology of stomatin

Stomatin is an integral membrane protein which is widely expressed in many cell types. It is accepted that stomatin has a unique hairpin-loop topology: it is anchored to the membrane with an N-terminal hydrophobic domain and the N- and C-termini are cytoplasmically localized. Stomatin is a prototype for a family of related proteins, containing among others MEC-2 (mechanosensory protein 2) from Caenorhabditis elegans, SLP (stomatin-like protein)-3 and podocin, all of which interact with ion channels to regulate their activity. Members of the stomatin family partly localize in DRMs (detergent-resistant membrane domains) enriched in cholesterol and sphingolipids. It has been proposed that a highly conserved proline residue in the middle of the hydrophobic domain directly binds cholesterol and that cholesterol binding is necessary for the regulation of ion channels. In the present study we show that a small part of the stomatin pool exists as a single-pass transmembrane protein rather than a hairpin-loop protein. The highly conserved proline residue is crucial for adopting the hairpin-loop topology: substitution of this proline residue by serine transfers the whole stomatin pool to the single-pass transmembrane form, which no longer localizes to DRMs. These results suggest that formation of the hairpin loop is inefficient and that the conserved proline residue is indispensable for formation of the hairpin loop. The single-pass transmembrane form exists also for SLP-3 and it should be considered that it mediates part of the physiological functions of stomatin and related proteins.

Vitamin C function in the brain: vital role of the ascorbate transporter SVCT2

Ascorbate (vitamin C) is a vital antioxidant molecule in the brain. However, it also has a number of other important functions, participating as a cofactor in several enzyme reactions, including catecholamine synthesis, collagen production, and regulation of HIF-1 alpha. Ascorbate is transported into the brain and neurons via the sodium-dependent vitamin C transporter 2 (SVCT2), which causes accumulation of ascorbate within cells against a concentration gradient. Dehydroascorbic acid, the oxidized form of ascorbate, is transported via glucose transporters of the GLUT family. Once in cells, it is rapidly reduced to ascorbate. The highest concentrations of ascorbate in the body are found in the brain and in neuroendocrine tissues such as adrenal, although the brain is the most difficult organ to deplete of ascorbate. Combined with regional asymmetry in ascorbate distribution within different brain areas, these facts suggest an important role for ascorbate in the brain. Ascorbate is proposed as a neuromodulator of glutamatergic, dopaminergic, cholinergic, and GABAergic transmission and related behaviors. Neurodegenerative diseases typically involve high levels of oxidative stress and thus ascorbate has been posited to have potential therapeutic roles against ischemic stroke, Alzheimer's disease, Parkinson's disease, and Huntington's disease.

The monovalent cation leak in overhydrated stomatocytic red blood cells results from amino acid substitutions in the Rh-associated glycoprotein

Overhydrated hereditary stomatocytosis (OHSt) is a rare dominantly inherited hemolytic anemia characterized by a profuse membrane leak to monovalent cations. Here, we show that OHSt red cell membranes contain slightly reduced amounts of Rh-associated glycoprotein (RhAG), a putative gas channel protein. DNA analysis revealed that the OHSt patients have 1 of 2 heterozygous mutations (t182g, t194c) in RHAG that lead to substitutions of 2 highly conserved amino acids (Ile61Arg, Phe65Ser). Unexpectedly, expression of wild-type RhAG in Xenopus laevis oocytes induced a monovalent cation leak; expression of the mutant RhAG proteins induced a leak about 6 times greater than that in wild type. RhAG belongs to the ammonium transporter family of proteins that form pore-like structures. We have modeled RhAG on the homologous Nitrosomonas europaea Rh50 protein and shown that these mutations are likely to lead to an opening of the pore. Although the function of RhAG remains controversial, this first report of functional RhAG mutations supports a role for RhAG as a cation pore.

The Glut1 and Glut4 glucose transporters are differentially expressed during perinatal and postnatal erythropoiesis

Glucose is a major source of energy for living organisms, and its transport in vertebrates is a universally conserved property. Of all cell lineages, human erythrocytes express the highest level of the Glut1 glucose transporter with more than 200 000 molecules per cell. However, we recently reported that erythrocyte Glut1 expression is a specific trait of vitamin C–deficient mammalian species, comprising only higher primates, guinea pigs, and fruit bats. Here, we show that in all other tested mammalian species, Glut1 was transiently expressed in erythrocytes during the neonatal period. Glut1 was up-regulated during the erythroblast stage of erythroid differentiation and was present on the vast majority of murine red blood cells (RBCs) at birth. Notably though, Glut1 was not induced in adult mice undergoing anemia-induced erythropoiesis, and under these conditions, the up-regulation of a distinct transporter, Glut4, was responsible for an increased glucose transport. Sp3 and Sp1 transcriptions factors have been proposed to regulate Glut1 transcription, and we find that the concomitant repression of Glut1 and induction of Glut4 was associated with a significantly augmented Sp3/Sp1 ratio. Glucose transporter expression patterns in mice and human erythrocytes are therefore distinct. In mice, there is a postnatal switch from Glut1 to Glut4, with Glut4 further up-regulated under anemic conditions.

Disturbance of cellular glucose transport by two prevalently used fluoroquinolone antibiotics ciprofloxacin and levofloxacin involves glucose transporter type 1

Dysglycemia and central nervous system (CNS) complications are the known adverse effects of fluoroquinolone antibiotics. Ciprofloxacin and levofloxacin are among the most prescribed antibiotics. In this study we demonstrate that ciprofloxacin and levofloxacin disturb glucose transport into HepG2 cells and such inhibition is associated with inhibited glucose transporter type 1 (GLUT1) function. When exposed to ciprofloxacin or levofloxacin at maximum plasma concentrations (C(max)) and 5x of C(max) concentrations, GLUT1 mRNA expression, cell surface GLUT1 protein expression and glucose uptake were significantly reduced. These findings imply that disturbed cellular glucose transport and GLUT1 function may underlie the dysglycemic and CNS effects of ciprofloxacin and levofloxacin.

The controversial place of vitamin C in cancer treatment

In 2008, we celebrate the 80th anniversary of the discovery of vitamin C. Since then, we know that vitamin C possesses few pharmacological actions although it is still perceived by the public as a "miracle-pill" capable to heal a variety of illnesses. Cancer is one of the most common diseases for which a beneficial role of vitamin C has been claimed. Thus, its dietary use has been proposed in cancer prevention for several years. Apart from this nutritional aspect, an extensive and often confusing literature exists about the use of vitamin C in cancer that has considerably discredited its use. Nevertheless, recent pharmacokinetic data suggest that pharmacologic concentrations of vitamin C can be achieved by intravenous injections. Since these concentrations exhibit anticancer activities in vitro, this raises the controversial question of the re-evaluation of vitamin C in cancer treatment. Therefore, the purpose of this commentary is to make a critical review of our current knowledge of vitamin C, focusing on the rationale that could support its use in cancer therapy.